The subject of the present invention is a tire composed of a plurality of mixes comprising as majority filler a non-conductive filler such as silica, or mixes having a low content of black filler, at least one of these mixes being the tread. It relates more particularly to a tire which may exhibit not insignificant internal temperature increases during travel, such as a tire intended, for example, to bear heavy loads. It also relates to a process for making such a tire, and an apparatus for carrying out such a process.
Since environmental problems are becoming ever more crucial, and savings in terms of fuel and the fight against the pollution produced by motor vehicles have become a priority, one of the aims of tire manufacturers is to produce a tire having all of the following: very low rolling resistance, excellent adhesion both on dry ground and on wet or snow-covered or icy ground, very good wear resistance, and finally reduced running noise.
To achieve this aim, a tire having a tread comprising silica as main reinforcing filler was proposed in European Patent Application No. EP A 501 227. Although this solution makes it possible to obtain the best compromise between all the very contradictory properties mentioned above, it has however proved that, depending on the vehicles, tires using a tread comprising silica as main reinforcing filler have the disadvantage of accumulating static electricity to a greater or lesser extent, this being formed by friction of the tire on the road during travel of the vehicle due to the lack of electrical conductivity of the silica.
The static electricity thus built up in a tire, when certain specific conditions are combined, is capable of giving the occupant of a vehicle an unpleasant electric shock when he has cause to touch the body of the vehicle. It is furthermore capable of accelerating ageing of the tire due to the ozone generated by the electrical discharge. It may also be the cause, depending on the type of ground and the vehicle, of poor operation of the onboard radio owing to the interference which it produces.
This problem of buildup of static electricity in a tire and of the majority of the disadvantages linked thereto is a very old one, and occurred even in the days when the reinforcing filler used was carbon black.
Application No. EP 0 658 452 A1 describes the adaptation of principles which have been known for a long time to a so-called “modern” tire, which adaptation makes it possible to solve the main problems pertaining to the solutions proposed in various older documents and, in particular, the harmful heterogeneity introduced into tire structures. The solution proposed consists in inserting a strip of conductive rubber mix or insert, extending preferably over the entire circumference of the tire and connecting the surface of the tread either to one of the crown plies, or to the carcass reinforcement, or to any other part of the tire which is sufficiently electrically conductive, the necessary electrical conductivity being imparted by the presence of a suitable carbon black.
U.S. Pat. No. 5,017,118 describes a co-extrusion apparatus comprising a single extruder which has a plurality of flow channels which open respectively on to an orifice formed by extrusion blades. Thus, as shown in FIGS. 7 and 8 of the document, the insert 120 parallel to the mixes 114, 118 and 122 arrives via a flow channel in the orifice 94 formed by the blades 106 and 98 on the surface 116 of the roller.
Although such a solution is perfectly viable for a tire comprising a tread formed of a single non-conductive mix, for example, a tread for a passenger vehicle, the same does not apply in the case of a tire comprising a plurality of layers of rubber mixes above the crown reinforcement and layers of rubber between the crown reinforcement and the carcass reinforcement, as is the case for any tire liable to travel at a high stabilized operating temperature, such as heavy vehicles or high-speed vehicles.
In fact, if for any reason one has cause to provide such a tire with a non-conductive layer, or inner part of the tread (part not in contact with the ground), between the crown reinforcement and the outer part (part in contact with the ground) of the tread which has been rendered conductive by the presence of a circumferential insert or striping, said inner part will have to be rendered conductive. Likewise, a layer between the carcass reinforcement and the crown reinforcement, having manifest oversize in the regions of the edges of crown plies, will also have to be rendered conductive if it is not initially so.
A first solution for obtaining a conductive tread consists in coextruding the inner and outer parts of the tread, and providing the assembly thus formed with a conductive circumferential insert. Said solution is not satisfactory for several reasons; two of these may be mentioned: in the type of tire in question, the total thickness of the tread is too great; furthermore, it may be advantageous for the conductive inserts, of the inner and outer layers of the tread respectively, not to be made with the same quality of rubber mix.
Another solution, as described in French Application No. FR 97/02276 by the Applicant, consists in providing the electrical connection between two layers which are or have been rendered conductive, separated by a non-conductive layer, by at least one strip of rubber mix of low thickness, width and length, laid between the two faces of the weld of the non-conductive layer and in contact with the means which render conductive the two layers connected by the connection. Although industrially satisfactory, said method requires additional laying of product and involves additional production costs.
A third solution consists in providing each non-conductive part with a circumferential insert having a circular rectilinear trace or circular striping after extrusion of said part by conventional extrusion means, and then joining the two products together before laying them on the crown reinforcement. To the extent that the striping is generally very fine in order not to upset the physical properties of the compositions constituting the two parts of the tread, the thickness of these inserts, on the tire viewed in cross-section, being of the order of 0.01 to 2.0 mm, such a solution requires the trace of the striping of the outer part of the tread on the contact wall between the two parts to be perfectly aligned with or centered on the trace on said wall of the striping of the inner part of the same tread.
Furthermore, the mechanical properties of the rubber compositions before vulcanizations are very poor, and the uncured rubber mix may be in the form of an extremely soft paste, or conversely an extremely hard agglomerate. Whatever the manner of working such mixes, it is difficult perfectly to control the geometry of the semi-finished product, the assembly of the two non-vulcanized parts of the tread: the concordance and alignment between the respective traces of the two sets of striping on the contact wall between the two parts cause great development difficulties, the solution of two sets of circular striping being unable to be considered as structurally optimized and industrially viable, both from the point of view of cost and of the performance obtained.
Thus although one of the aims of the invention is, in a tire having a plurality of non-electrically conductive mixes, to dissipate the electrostatic charges induced by the travel of the tire, without significantly affecting the level of the properties of the tire, the other aim is to be able to obtain a tire as simply as possible and at lower cost, be these material costs and/or manufacturing costs.